This invention relates to an ignition timing control system for internal combustion engines.
An ignition timing control system of an internal combustion engine in general is adapted to regulate the timing at which high-voltage pulses are supplied to the engine spark plugs in dependence upon a change in engine rpm in order to achieve optimum combustion of fuel in the engine cylinders, and to prevent the occurrence of a reverse torque when the engine runs in a region of very low rpm.
An ignition timing control control system for an internal combustion engine includes a pulser for generating first and second timing pulses, which are positive and negative, respectively, at predetermined crank angle positions close to top dead center of an engine cylinder when the engine rotates. For reasons that will be described in detail later, the maximum delay angle position of ignition timing at low engine speed corresponds to the trailing edge of the second timing pulse which is influenced by the rotational speed of the engine and becomes unstable particularly when the engine is rotating at very low speed, as when the engine is started. With the conventional ignition timing control system, therefore, the maximum delay angle position for ignition timing corresponds to the instant at which the second timing pulse decays and, hence, is influenced by the width of this pulse which, as mentioned above, is affected by engine speed, especially in the low-speed region. As a result, ignition timing at the maximum delay angle can become unstable and it may be difficult to suppress the occurrence of reverse torque in the engine. This makes it necessary to increase the mechanical strength of the engine, which forms a cause of greater engine weight.
In another aspect of an internal combustion engine ignition system, a capacitative-discharge ignition unit (commonly referred to as a C.D.I unit) is often employed in place of a mechanical breaker contact to prevent failures and deterioration in the overall ignition system. The C.D.I. unit includes a generator (ACG) having a charging generator coil (hereafter referred to as an exciter coil) the output whereof charges a capacitor in an ignition circuit, and an ignition signal-forming coil (hereafter referred to as a pulser coil, which is wound on the above-mentioned pulser) for producing an ignition signal applied to an ignition signal generating circuit. The ignition signal generating circuit produces an ignition signal at a predetermined timing dependent upon the rotational speed of the engine, and the ignition signal turns on a thyristor (SCR) connected to the abovementioned capacitor to discharge the same. This causes a large current to flow into the primary coil of an ignition coil connected to the capacitor, thereby producing a high voltage in the secondary coil of the ignition coil. The high voltage causes a spark plug to produce a spark. Ignition timing in an arrangement of this type ordinarily is controlled by a phase difference between the aforementioned first (positive) and second (negative) timing pulse signals produced by the pulser coil.
A conventional ignition timing control system employing such a C.D.I. unit is designed to be free of the effects of power supply fluctuation, pulser gap variance and changes in temperature in order to achieve accurate ignition timing and prevent the occurrence of reverse torque. However, this is achieved at the expense of circuit simplicity, compactness and ease of assembly, since a large number of circuit elements are required. There is thus room for improvement.